1. Field of the Invention
The present invention relates to an electrophotographic toner composition for developing an electrostatic latent image in electrophotography or electrostatic printing. The present invention also relates to a method of producing the toner.
2. Discussion of Background
In electrophotography or electrostatic printing based on the Carlson process, electrophotographic developers are generally employed for visualizing an electrostatic latent image. These developers are broadly divided into two groups. In one group are two-component developers which are each composed of a toner and a carrier admixed such that an appropriate electrostatic charging is attained by donating charges onto the toner, as disclosed in Japanese Laid-Open Patent Application (referred to hereinafter as JPA) No. 61-147261, for example. In the other group are one-component developers which are each composed of a toner, and with which charging is carried out on a roller for supplying toner in a developing sleeve portion by transferring charges from a blade.
The toners are prepared as minute particles, including either synthetic or natural thermoplastic resin as a major component, in which is dispersed coloring agents such as carbon black or dye material. The toner particles are generally provided with a variety of additive agents to achieve proper and stable charging characteristics durable for a prolonged period of time.
Since two-component developers are electrostatically charged by transferring charges onto the toner, as described earlier, it is of major importance for the toner to retain a stable and predetermined amount of charge throughout imaging processes, to thereby achieve and also to retain reliable and durable imaging capabilities. In addition, since the charging is achieved in the two-component developers typically through triboelectric effects induced by the collision and friction between toner particles, charging is affected primarily by the balance between the charging and charge retaining capabilities of the toner particle. It has been known that this balance between charging and charge retaining is largely dictated by the dielectric properties of toner particles.
As requirements for electrophotographic images with more minute and more reproducible features have increased, the need for toners with greater fluidity has also increased. Several attempts to improve toner fluidity are known, such as by the addition of hydrophobic fine powders, as disclosed in JPA-52-30437 and 60-238847. However, when hydrophobic powders are added to toner in amounts sufficient to achieve desired toner fluidity characteristics, these hydrophobic powders adversely affect the stability of the charging property of the toner. In addition, as the size of the toner particles decreases, the amount of hydrophobic powders that must be added to the toner increases, in general.
Furthermore, in place of the prior method of charging, in which the charging of a photoreceptor is carried out through a corona discharge induced by high voltages applied to a metal wire, an alternative method has been attracting attention. In this alternative method, an image bearing member such as a photoreceptor is brought into close contact with an image forming member such as an image transfer sheet or copy sheet, and the charging is then carried out using an electrode which is in contact with a transferring member, to thereby induce a voltage potential therebetween. Since the latter method is able to transfer toner particles by pressure as well as the voltage potential, this method gives rise to a decrease in the amount of ozone generated during the toner transfer process, which is preferable over prior methods.
One of the shortcomings of the latter method is that the minute line images formed by the method contain missing portions. Although this shortcoming can be obviated to some extent by the use of fluid toner particles, a relatively large amount of additive agents is generally required, often resulting in a decrease in durability of toner charging capability and also in image quality caused by an increase in the amount of charge with time elapsed. Toner particles characterized by the addition of these additive agents are disclosed in JPA-7-56380 and 7-230179, for example, which use titanium oxide particles which are surface treated with hydrophobic agents such as silane, silica or silicone compounds. These surface treated titanium oxide particles, however, are not as satisfactory as hydrophobic silica particles in solving the above-noted difficulty encountered in the improvement of the toner fluidity. In addition, these surface treated particles, in general, may result in the decrease in charging capability caused by high dielectric and low resistivity properties of toner particles themselves.